Arylazotetracyanocyclopentadienides

The phenomenon of photochromism, a reversible photoreaction, has been observed on a huge variety of molecules.¹⁹² Despite great effort to find new design principles for photoswitches, only few classes dominate the field, with azobenzenes probably being the most prominent one. On the one hand, azobenzenes have been applied to various fields, such as life science,¹⁴ materials science,¹⁰ catalysis,¹² and molecular machines.¹³ On the other hand, tunability of the photochromic properties has seen many advances during the last decade, by exploring the effect of functional groups, especially in the ortho position,²²² heteroaromatic substituents,²³⁷ and coordination²³¹/protonation²²⁹ of the azo bond.

Several problems still remain for many derivatives, namely most azobenzenes are limited to nonpolar solvents and require a solubilizing group if water is within the scope of the application. Furthermore, the low extinction coefficient of the n-π* band²⁴⁷ often requires long irradiation times or high light intensities, which may result in the use of UV-light for switching via the much more intense π-π* transition. Usually the π-π* bands are well separated, allowing for efficient E to Z switching with UV-light, whereas the n-π* bands in the visible region largely overlap, resulting in lower photostationary states for the Z to E isomerization. Here, a new design for azobenzenes is introduced, which relies on the tetracyanocyclopentadienide (TCCp) substituent and does not suffer from the aforementioned drawbacks.

Webster found in 1965 that the dianion of hexacyanobutadiene 94 would cyclize with acid to form aminoTCCp 95, which he treated with NaNO2 to prepare the TCCp diazonium salt 96. It is a remarkable compound in terms of thermal stability (decomposition > 200 °C) and dipole moment (11.4 D),²⁴⁸ which can undergo azo coupling to electron-rich aromatic systems.²⁴⁴

Thereafter, the TCCp structural motive has been used in supramolecular chemistry,^249,250 but the photochromic properties of these azo dyes have not been reported, until now. The only compound where a cyclopentadienyl moiety is connected to an azo bond and for which photoreactions have been investigated is azoferrocene, where the isomerization to the Z isomer is irreversible.²⁵¹ It was surprising that the already known azo dye 97E undergoes a reversible photoreaction and hence this interesting class of molecules has been examined (Scheme 62 and Figure 34).

NC

Scheme 62: T-type photochromism of an arylazoTCCp switch.

73 Figure 34: UV/vis absorption spectra of 97E and the PSS after irradiation with 405 nm and 436 nm in acetonitrile (1.6 ∙ 10⁻⁵ M, −20 °C). The spectrum of 97Z can be derived from the PSS spectra and the pure E spectrum

according to the method developed by Fischer.⁴⁴

Mixing the TCCp diazonium salt 96 and an electron-rich aromatic molecule in acetonitrile usually results in quantitative azo coupling, while the main losses during synthesis result from purification by recrystallization, especially on a small scale. To enable a broader substrate scope, Mills coupling conditions have been investigated, which provide access to electron poor aromatic molecules, such as benzonitrile or nitrobenzene, as well (Scheme 63).

1) Oxone (water/CH2Cl₂) 2) (AcOH)

3) NEt4Cl (water) Na⁺

NC

NC CN

CN

NH₂ CN

NH₂ NC

NC CN

CN N N

CN

+

NEt₄⁺

73%

95 98 Scheme 63: Mills coupling reaction to introduce electron-deficient substituents.

Most of the arylazoTCCps are crystalline solids and single crystal structures for several derivatives have been obtained (Figure 35). For the potassium salt 103, coordination of the potassium cation is found for methoxy O, azo N, and nitrile N. The latter is typical for pentacyanocyclopentadienide salts (Figure 36).²⁵²

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Figure 35: Molecular structures of some arylazoTCCp switches. Ellipsoids are set at a 50% probability. Top-left:

98; top-right: 99; middle-left: 100; middle-right: 101; bottom: 102.^vi

vi X-Ray analysis of 98, 100, and 102 performed by Florian Q. Römpp. X-Ray analysis of 99 and 101 performed by Bernd M. Schmidt.

75 Figure 36: Molecular structure of the potassium salt of 103. Ellipsoids are set at a 50% probability.^vii

Since some of the newly synthesized photoswitches show positive photochromism in combination with a fast thermal back reaction, the standard methods (UV/vis in combination with NMR or HPLC) for obtaining the spectrum of the pure Z isomer are problematic to use, since they would have to be done at lower temperatures. To solve this classic problem of photochromism, the arylazoTCCp switches have been analyzed by applying the Fischer method⁴⁴ at temperatures, where the speed of the thermal back reaction is neglectable compared to the speed of the photoreaction. Assuming that the quantum yields for both isomerizations are independent of the irradiation wavelength, the spectrum of the pure Z isomer can be obtained from the pure E spectrum and the photostationary state (PSS) spectra at two different wavelengths. To verify the quantum yield assumption, three instead of two different wavelengths have been used for monochromatic irradiation (365, 405, and 436 nm), which resulted in similar Z spectra for each pair, with few exceptions due to intrinsic limitations of the method (see experimental section 5.3) or irradiation to a higher excited state.

Surprisingly, the TCCp unit leads to a large separation between the absorption maxima of the bands in the visible region for E and Z isomer of up to 80 nm. So far, only the bridged Z azobenzenes from the group of Herges show a similar large band separation.^234,235 For most azobenzenes the n-π* bands overlap, which makes the use of UV-light for switching via the π-π* band necessary. A large band separation is a prerequisite for a high PSS, since it allows to irradiate both isomers almost exclusively.

In general, the Z isomers of arylazoTCCp switches show only little absorption around 405 nm, resulting in PSSs with 90% Z content. For the back reaction red light with a cut-off filter or heat can be used, allowing for almost full photoconversion in both directions.

Some of the photochromic properties are listed in Table 4 and some trends are found comparing to the benzene analogs: (1) All of the arylazoTCCps switch efficiently with visible light in both directions, resulting in a PSS around 90% for most derivatives. (2) The extinction coefficients of the E isomers in the visible are generally higher (> 20000 L mol⁻¹cm⁻¹) than for classic azobenzenes. (3) The E isomers show fine structure in the UV/vis spectrum, whereas the Z isomers lack such vibronic features. (4) The thermal half-lives are shorter compared to classic azobenzenes, ranging from few minutes to 14 h. (5) The quantum yields for both reactions are relatively high (0.2-0.7), typically the Z to E quantum yield being the higher one. (6) No fatigue was observed under prolonged irradiation, with the only exception being the free pyrrole derivative 111. (7) The established design rules for azobenzene apply as well for TCCp azos: Strong donors (N,N-dimethylamino, 97) or strong acceptors (nitro, 104) lower the thermal

vii X-Ray analysis of 103 performed by Bernd M. Schmidt.

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stability of the Z isomer.²⁵³ Fluorine atoms in the ortho-position of the benzene ring can be used to increase the thermal half-life²⁵⁴ (compare 97 vs 107 or 106 vs 99), whereas hydroxyl groups, either in ortho or para, lead to low conversions under irradiation (108, 109, 110).²⁵⁵

It was possible to synthesize heteroarene azos, which resulted in longer thermal half-lives, e.g. 8 h for pyrrazole 112 or 13 h for N-methylpyrrole 102. The free pyrrole 111 is found to be photochromic with a thermal half-life around 3 min, although prolonged irradiation led to side products, which prohibits a detailed analysis by the Fischer method.

Table 4: Photochromic properties of the new tetracyanocyclopentadienyl azo-compounds.^viii No. aryl substituent λmax E (ε)

viii Spectroscopy of 97, 98, 101, 104, 105, and 106 together with Jonas Becker.

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g No suitable wavelengths for the reverse reaction has been found, since the spectra show substantial overlap.

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The common approach to solubilize a compound in a solvent where the solubility is low is the attachment of a solubilizing group. Depending on the polarity of the solvent one would choose different groups, e.g. alkyl chains for hexane and glyme for water. In general, this results in different molecules which are not necessarily comparable. The beauty of ionic compounds lies in the solubility tuning by proper choice of the counter-ion, leaving the functional part untouched. To exemplify the variety of applicable solvents, the K⁺ salt as well as the Bu4N⁺ salt of 107 have been prepared and were investigated in nonpolar (THF) and polar (water) solvents, as well as in an ionic liquid (1-butyl-3-methylimidazolium tetrafluroborate, Figure 37). A small bathochromic shift is observed going to more polar solvents for both isomers. The band in the visible region seems to feature at least two different transitions, of which the lower energy transition has a higher extinction coefficient in polar solvents. The higher energy transition has a higher extinction coefficient in nonpolar solvents while the corresponding band only appears as a weak shoulder in water. This results in a larger band separation in nonpolar solvents. Furthermore, polar solvents accelerate the thermal back reaction, which allowed to tune the thermal half-life of 107 from 91 min (THF) to 2.1 s (water).

Figure 37: UV/vis absorption spectra of 107 in various solvents before and under irradiation (normalized to 2 ∙ 10⁻⁵ M): A bathochromic shift is observed going from nonpolar to polar solvents for both isomers, [BMIM]BF4

is 1-butyl-3-methylimidazolium tetrafluroborate.

Covalent attachment of the cation to the aryl substituent would result in zwitterionic molecules.

Zwitterions^256,257 are widely applied as solubilizing groups^258,259 and for the extraction of heavy metal ions from drinking water by chelation, as well as for sewage treatment, paper reinforcement, pigment retention, and formulation in shampoos or hair conditioners.²⁶⁰ They also resist to nonspecific protein binding, which makes them applicable as nonfouling materials.^261–263 Such materials have various further microbiological applications²⁶⁴ and can be utilized to produce “stealth” nanoparticles for drug delivery.²⁶⁵ Zwitterionic compounds have proven to increase the conductivity in lithium metal batteries, by facilitating the dissociation of lithium ions from the polyelectrolyte and thereby increasing ion mobility.^266,267 The trimethylammonium switch 105, which is easily obtained by methylation of the dimethylaniline derivative 97 with methyl iodide is a switchable zwitterion (Figure 38). Upon irradiation, the distance between positive and negative charge changes, which results in a change of the dipole moment and could be used in the context of some of the aforementioned applications.

79 Figure 38: UV/vis absorption spectra of 105E and the PSS after irradiation with 365 nm, 405 nm and 436 nm in acetonitrile (1.7 ∙ 10⁻⁵ M, 25 °C). The spectrum of 105Z can be derived from the PSS spectra and the pure E spectrum according to the method developed by Fischer.⁴⁴

During their investigations on ortho-substituted azobenzenes, the group of Woolley found that the thermal half-life of photochromic azonium ions is in the range of microseconds to milliseconds when ortho-methoxy groups are introduced.²⁶⁸ The arylazoTCCp derived from phloroglucinol trimethylether 101, which bears two ortho-methoxy groups as well, exhibits a large hyperchromic and bathochromic shift upon protonation from 377 to 530 nm. The protonation supposedly occurs at the azo bond, given the extremely large pKa value of the TCCp unit itself²⁶⁹ and results in the zwitterionic (and therefore overall charge-neutral) azonium species (Scheme 64). Under irradiation with green light (546 nm) the Z isomer forms and reverts back with an astoundingly long thermal half-life of 2.1 min at room temperature. Furthermore, upon protonation a hyperchromic shift is observed for both isomers, which results in a surprisingly high extinction coefficient of the Z isomer (24600 L mol⁻¹cm⁻¹, Figure 39).

NC

NC CN

CN N -N O

O O

H NC

NC CN

CN N N

O NEt₄⁺

O

O +H⁺

-NEt₄^{+ +}

101 101-H⁺

Scheme 64: Protonation of 101 causes a bathochromic shift and results in surprisingly long thermal half-life of the Z isomer.

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Figure 39: Photochromism of 101-H⁺ in acetonitrile (1.6 ∙ 10⁻⁵ M, −20 °C). An excess of methanesulfonic acid was added to assure complete protonation. The thermal half-life is 2.1 min (25 °C). The pure Z spectrum was calculated according to the method developed by Fischer.⁴⁴

In conclusion, the TCCp serves as a superb substitute for the benzene ring in azo-based photoswitches.

A broad structural variation is possible, including electron-rich or electron poor benzene derivatives, as well as heterocycles. The TCCp unit provides a handle to overcome the problem of the poorly separated n-π* bands in the visible region, where band separations of up to 80 nm have been observed. The combination of absorption maxima with high extinction coefficients (> 20000 L mol⁻¹cm⁻¹) for the E isomer together with the absorption gap of the corresponding Z isomer at almost the same wavelength assures high photostationary states utilizing only visible light for both isomerization directions. Given the anionic nature of the TCCp unit, the solubility of the same switch can be tuned from THF to water (and even to ionic liquids), which is accompanied by a change of thermal half-life from 1.5 h to 2 s going to more polar solvents and may lead to applications in switchable ionic liquid crystals.²⁷⁰ By covalent attachment of the cation to the aryl unit, a photochromic zwitterion has been shown to switch efficiently between both isomers, which has potential use in material and biological applications. Furthermore, the TCCp unit stabilizes the Z isomer of the corresponding azonium species in acidic media, giving rise to azo-based pKa switches, which do not suffer from a fast thermal back reaction.

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